1. Field of the Invention
The present invention relates to a method for fabricating a bulk acoustic resonator, and more particularly, to a bulk acoustic resonator fabricated with a dimple in a surface of a resonance part positioned above a cavity formed in a substrate so as to adjust a direction toward which the resonance part vibrates, and a method for fabricating the bulk acoustic resonator.
2. Description of the Related Art
With the rapid popularization of mobile communication devices, representative of which are portable phones, demands for more compact and lighter filters for use in the mobile communication devices have been sharply increased. Bulk acoustic resonators are known to be compact and light filters. Bulk acoustic resonators can be mass-produced in a small form factor at a minimum cost. Also, the bulk acoustic resonators can realize high quality factor (Q) values that are major characteristics of filters and be used in a microwave frequency band, particularly, in personal communication system (PCS) and digital cordless system (DCS) bands.
In general, a bulk acoustic resonator's resonance part is made by sequentially stacking a first electrode, a piezoelectric layer, and a second electrode on a substrate. If electric energy is applied to the first and second electrodes to maintain an electric field in the piezoelectric layer, the electric field causes a piezoelectric phenomenon in the piezoelectric layer so as to vibrate the resonance part. Thus, a bulk acoustic wave is generated in the same direction as the direction along which the resonance part vibrates so as to produce a resonance.
Examples of such a bulk acoustic resonator include a Bragg reflector type resonator and an air gap type resonator. In the Bragg reflector type resonator, stepped materials having a great elastic impedance difference are deposited on a substrate to constitute a reflector layer, and a lower electrode, a piezoelectric layer, and an upper electrode are sequentially stacked on the reflector layer. Thus, elastic wave energy having passed through the piezoelectric layer is not transmitted toward the substrate but reflected by the reflector layer so as to produce an efficient resonance. However, it is difficult to fabricate the reflector layer for totally reflecting the elastic wave energy in the Bragg reflector type resonator. Thus, a large amount of time and cost are required in fabricating the Bragg reflector type resonator.
To solve the disadvantages of the Bragg reflector resonator, an air gap resonator uses an air gap instead of a reflector layer so as to isolate the resonance part from the substrate. Thus, the resonator has a high reflection characteristic and a stable practical band.
FIG. 1 is cross-sectional view of a general air gap type resonator. Referring to FIG. 1, the general air gap type resonator includes a resonance part 60 positioned on a substrate 10. A cavity 50 is formed in a surface of the substrate 10 so as to isolate the resonance part 60 from the substrate 10. The resonance part 60 includes a structure in which a lower electrode 20, a piezoelectric layer 30, and an upper electrode 40 are sequentially stacked.
In the general air gap type resonator shown in FIG. 1, the resonance part 60 is fabricated in a flat board form in a space above the cavity 50. In general, the resonance part 60 fabricated in the space above the cavity 50 is circular or polygonal as viewed in a horizontal direction of the air gap type resonator. If a position of a power source connected to the lower and upper electrodes 20 and 40 is changed in this case, a direction along which an electric field is formed between the lower and upper electrodes 20 and 40 varies. A vibration direction of the resonance part 60 varies with the variation in the direction along which the electric field is formed. In other words, the electric field may pass through the center or edge of the resonance part 60.
An area in which the electric field is directly formed above the resonance 60 experiences a great vibration due to a great intensity of the electric field, while the other areas have relatively small vibration. Thus, the vibration direction varies depending on where the electric field is formed.
The resonance part 60 may vibrate perpendicular to the surface of the substrate 10. However, in the general air gap type resonator, the resonance part 60 is fabricated in flat board form. Thus, the position of the power source connected to the lower and upper electrodes 20 and 40 can be adjusted so that the resonance part 60 vibrates perpendicular to the surface of the substrate 10. As a result, it is difficult to design an optimal resonator.